A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of one, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In current designs for lithographic apparatus operating in the EUV range (with wavelengths typically of 5-20 nm) of the electromagnetic spectrum, highly reflective elements are typically provided in order to condition and pattern a beam for transferring a pattern from a patterning device onto a substrate. This part of the electromagnetic spectrum is sensitive to transmission losses since the radiation is easily absorbed by most surfaces. In order to increase the reflectivity, mirror elements have been developed typically comprising a ruthenium top layer (for example Ru-capped multi-layer mirrors and Ru grazing incidence mirrors). Furthermore, the sources that are used to produce such EUV radiation are typically plasma sources, wherein current designs in particular use a tin source. These plasma sources have a tendency to produce, in addition to EUV radiation, a variety of debris particles, which may, without appropriate measures, migrate into the system and cause contamination and malfunction.
In particular, tin contamination from the plasma source will deposit on mirror elements and cause serious loss of reflectivity of the mirror. This is prevented by contraptions to catch the debris before it can do any harm. However, these contraptions can have a certain fail ratio. Typically, such a contraption is a so-called foil trap which is designed to trap debris particles in substantially aligned foil plates. Downstream, relative to the direction of EUV radiation, a collector is typically arranged to collect an optimal portion of radiation coming out of the EUV-source. Such a collector is typically an arrangement of mirror elements, which use a grazing incidence reflection for collecting and guiding incident radiation into a beam of EUV radiation. In particular, for the mirror elements of the collector, but also for other mirror elements which are critically subject to contamination, it is therefore desirable to provide cleaning techniques to clean these mirror elements from contamination. One of such techniques is so called hydrogen cleaning. In this method, in particular in combination with a tin source, hydrogen radicals react with tin to form gaseous tin-hydrides (SnH4). Another technique is halogen cleaning. Also, a combined technique may be used, in particular, wherein tin oxides are reduced to tin using hydrogen, and where the thus formed tin is removed using halogen cleaning. However it has been found that for typical mirror elements, in particular comprising ruthenium, the cleaning rate drops dramatically for very thin tin deposits. Such thin contaminant deposits may degrade the transmission of the EUV mirrors and may be hard to remove. For hydrogen cleaning, removal of such thin deposits may lead to unacceptable down time for the lithographic apparatus.